The present invention relates to methods and apparatus for improving process results while processing a substrate in a plasma processing system.
Plasma processing systems have been around for some time. Over the years, plasma processing systems utilizing inductively coupled plasma sources, electron cyclotron resonance (ECR) sources, capacitive sources, and the like, have been introduced and employed to various degrees to process semiconductor substrates and glass panels.
During processing, multiple deposition and/or etching steps are typically employed. During deposition, materials are deposited onto a substrate surface (such as the surface of a glass panel or a wafer). For example, deposited layers comprising various forms of silicon, silicon dioxide, silicon nitride, metals and the like may be formed on the surface of the substrate. Conversely, etching may be employed to selectively remove materials from predefined areas on the substrate surface. For example, etched features such as vias, contacts, or trenches may be formed in the layers of the substrate. Some etch processes may utilize chemistries and/or parameters that simultaneously etch and deposit films on the plasma-facing surfaces.
The plasma can be generated and/or sustained using a variety of plasma generation methods, including inductively-coupled, ECR, microwave and capacitively-coupled plasma methods. Plasma processing systems differ widely in their configurations, some of which may be more suitable for certain etch processes than others.
Plasma processing systems employing clamping rings have been employed for some time to etch a variety of features in semiconductor substrates. To facilitate discussion, FIG. 1 illustrates a cross-section view of an exemplary plasma processing system 102. Plasma processing system 102 includes a chamber wall 104, which provides an enclosure and defines a processing chamber therein as well as defining an exhaust passageway 106 for exhausting etch byproducts. Chamber wall 104 is grounded in the exemplary plasma processing system of FIG. 1.
An upper electrode 108, which is also electrically grounded in the example of FIG. 1, functions as the etchant source gas distribution mechanism. Etchant source gas is introduced into the chamber via an inlet 112 and is distributed in the plasma region 114 between upper electrode 108 and a substrate 114, which is disposed above a lower electrode 116.
Lower electrode 116 is energized by a RF generator 120, which provides RF power to lower electrode 116 via a match network 122. A V/I probe 124 is shown coupled to match network 122 to measure the voltage and current furnished by RF generator 120 for feedback control purposes. In the example of FIG. 1, RF generator 120 supplies both 2 MHz and 27 MHz RF frequencies to lower electrode 116. When RF generator 120 supplies RF power to lower electrode 116, a plasma is ignited and sustained in the aforementioned plasma region 114 to etch substrate 124.
Lower electrode 116 is secured to chamber wall 104 via a lower electrode securing arrangement that includes O-rings 130A-B, an insulator ring 132, a clamping ring 134, and a set of stainless screws, of which two screws 136 and 138 are shown. Clamping ring 134 is typically formed from an insulator material, such as ceramic, to insulate lower electrode 116 from grounded chamber wall 104. As shown in FIG. 1, clamping ring 134 includes a shoulder 140 that bears against a lip 142 of chamber wall 104 when screw 136 is tightened to pull lower electrode 116 downward toward clamping ring 134. With clamping ring 134 held immobile by lip 142 and shoulder 140, the tightening of screw 136 causes lower electrode 116 to compress O-rings 130A-B against insulator ring 132 and chamber wall 104, thereby forming a tight seal to isolate the chamber interior from the ambient environment.
FIG. 1 also shows an upper focus ring 150 and a lower focus ring 152, which help focus and maintain the plasma in the region above substrate 124 to improve process results. The components of FIG. 1 are conventional and may be found in, for example, the Exelan™ family of plasma etchers, which is available from Lam Research Corporation of Fremont, CA.
FIG. 2 shows clamping ring 134 in greater detail, including shoulder 140 and a plurality of holes 160 to accommodate the aforementioned screws, such as screw 136 or 138 of FIG. 1.
FIG. 3 shows the lower electrode securing arrangement in greater detail. Screw 136, which is typically formed of stainless steel or another suitable relatively non-reactive high strength material, is disposed in a cavity created within clamping ring 134. Clamping ring 134, which may be formed of for example alumina or another suitable ceramic material, is insulated from screw 136 by a clamping ring sleeve 302, which lines the interior of the clamping ring cavity. Clamping ring sleeve 302 may be formed from a suitable plastic material such as VESPEL, DELRIN, or PEEK, and is provided to help reduce the corona electrical discharge during processing between the high RF voltage surfaces of screw 136 (which is coupled to lower electrode 116) and the grounded surfaces of chamber wall 104.
As shown in FIG. 3, the cavity within clamping ring 134 is counter-sunk to accommodate a screw head 144 of screw 136 completely within the body of clamping ring 134 while allowing screw head 144 to bear against clamping ring 134 when screw 136 is tightened against lower electrode 116. Since clamping ring 134 is held immobile by the aforementioned shoulder 140 of clamping ring 134 and lip 142 of chamber wall 104, such tightening of screw 136 forces lower electrode 116 downward, thereby compressing O-rings 130A-B to form a tight seal.
Even though screw head 144 of screw 136 is completely disposed inside the body of clamping ring 134, the likelihood for discharge from the screw head is further reduced by the use of a clamping ring cap 304, which screws into the threaded portion of clamping ring sleeve 302 to reduce the volume of empty space 146 that exists within the clamping ring cavity.
It has been discovered, however, that clamping ring sleeve 302 and clamping ring cap 304 in the lower electrode securing arrangement of FIG. 3 fail to prevent damaging electrical discharge under certain process conditions. For example, inspection of clamping ring sleeve 302 and clamping ring cap 304 in field-deployed plasma processing systems reveals arching failure, including burning, of certain regions of clamping ring sleeve 302 and clamping ring cap 304. The sleeve regions where the damage tends to be observed are marked in FIG. 3 by reference numbers 310 and 312. These regions are typically adjacent to sharp corners of screw 136, and a close inspection of the damaged sleeve regions suggests that RF energy has been absorbed by the damaged sleeve regions.
While not wishing to be bound by theory, it is believed that when the plasma acts as the principal resistive component during processing. When the plasma is extinguished (e.g., due to certain operating conditions that no longer sustain the plasma), the resistive component of the plasma chamber is substantially eliminated and the capacitive component dominates. As power continues to be furnished by the RF generator, little or none is absorbed by the resistive component. This condition in turn causes the RF voltage to increase since the match network of the RF power supply system will attempt to increase the RF voltage to match the power absorption with the amount of power that the RF generator has been asked to provide.
The increased RF voltage in turn increases the strength of the electric field between screw 136 and the grounded surfaces of the chamber wall 104. At some later point in time, the RF generator is turned off, and the electric field strength is reduced. However, during the window of time between the time the plasma is extinguished and the time the RF generator is turned off, a high electric field exists between the screws and grounded surfaces of the chamber wall, causing discharge damage to some regions of the clamping ring sleeve 302 and/or clamping ring cap 304.
As RF energy is absorbed the clamping ring sleeve/cap, the clamping ring sleeve/cap is damaged and begins to lose some of its capacitance and/or insulative qualities. Because of the damage to the clamping ring sleeve, less RF power is delivered to the plasma during subsequent etches. The degree of sleeve/cap damage changes over time, resulting in a change in the amount of power delivered to the plasma over time. This variation in the amount of RF power delivered to the plasma results in inconsistent etches from substrate to substrate, disadvantageously increasing defects and lowering yield.